1. Field of the Invention
The present application relates to slowing or stopping the vibratory speed of an ultrasonic needle tip of a surgical instrument during ocular surgery or to withdrawing the needle, depending upon whether a medically unsafe thermal condition is likely to be reached if the ultrasonic needle tip is permitted to continue to vibrate at the same rate of speed and at the same location.
2. Description of Related Art
U.S. Pat. No. 5,409,481 describes a laser tissue welding control that includes monitoring through a surgical microscope and using an infrared radiation wavelength detector, i.e., a pyrometer, to ascertain temperature of a viewed object because the sensed infrared wavelengths are proportional to the temperature of the viewed object. An example of a pyrometer is that of Model M67S produced by Mikron Instrument Co.
U.S. Pat. No. 5,505,693 (the '693 patent) describes an invention that reduces heat generation during ocular surgery from giving rise to an medically unsafe thermal condition caused by heat generation from frictional effects of a vibratory motion of an ultrasonic needle tip of a surgical instrument acting on surrounding tissue. The normal temperature of body tissues is 37° C., the surface tissue of the eye is normally slightly cooler, typically 35° C., and a temperature of approximately 55° C. or greater can cause damage to ocular tissue.
In a worst-case scenario of an ultrasonic transducer, driver and needle with a mass of 23 grams, a frequency of 60 KHz and a stroke length of 0.004 inches, the following calculation can be made. Ultrasonic power is approximately 32 Joules/second. If 80% of this energy is dissipated on the sleeve(s), the heat energy released would be 6 calories/second. Assuming that the area of a sleeve in contact with the tissue is 15-20 square millimeters and that a 3 mm thick region of tissue surrounding the sleeve accepts all the heat, temperature rise (in this region of tissue) would be 10°.-14°. C./second. Within this region of tissue and fluids, there will exist a temperature gradient, with the tissue in direct contact with the sleeve having the highest temperatures, and that most separated from direct sleeve contact experiencing lesser temperature elevations.
Under these circumstances, the approximate 55° C. or greater limit would be reached in 1.5-2.0 seconds of full-power application by the ultrasonic transducer. In accordance with the invention, a dynamic friction coefficient of 0.1 between the outer needle surface and the inner sleeve surface will reduce heat generation by 90%, and will allow at least 15-20 seconds of operation before a tissue temperature of approximately 55° C. or greater limit is reached.
It would be desirable to prevent a medically unsafe thermal condition from being reached while operating a vibratory needle of a surgical instrument.